The present invention relates to an optical method and system for providing precise real time location in extended three-dimensional space of a movable point; that is, a point which is generally in motion, such as a point which is located on a robotic end effector.
Precise location of parts which move, either in three-dimensional space or merely along a line, is necessary in many applications. Examples include work-performing tools and robotic end effectors. More particularly, precise location of robotic end effectors to an accuracy of at least one mil (1.times.10.sup.-3 inch) over a volume of several feet in each dimension is highly desirable in order to provide feedback for accurate metrology, machining and assembly operation. Previous approaches to this problem do not provide the combination of accuracy, working volume, time response, and non-contact remote operation needed for robotic end effector control.
Typically, location of robotic end effectors is accomplished in a mechanical approach by determining the angles of various joints which permit end effector motion and then applying geometrical calculations. However, as a practical matter errors in angle determination, as well as compliance of the joints and limbs, often limit reliable mechanical location to about ten mils accuracy, which is much poorer than the one mil accuracy needed in many cases. Furthermore, the required limb sitffness for even this degree of accuracy adds undesirable weight to the limbs.
Optical approaches to this same problem have also been developed, based, for example, on interferometric and triangulation techniques.
By way of a more particular example, optical interferometric approaches utilize striped patterns which result from the interference of coherent beams of light. As is known, if the beams from a pair of spaced coherent light sources, such as produced by a laser, are projected in generally the same direction, constructive and destructive light wave interference between the two beams produces a striped pattern of alternating light and dark fringes. The separation between the fringes is highly predictable, and is a function of the wavelength of the light involved and of the angle between the two coherent light sources as viewed from any particular point in the region where the light beams intersect.
Thus, in general, it is known to monitor movement across a set of such interference fringes by counting the number of fringes crossed. However, there are a number of disadvantages to such a basic approach. For example, counting fringes alone does not indicate the direction of movement.
Another disadvantage is that ultimate resolution is dependent upon the spacing between fringes, as well as upon the particular technique employed for interpolating position between fringes. Increasing resolution by increasing the fineness of the fringes, such as by decreasing the fringe spacing, is subject to a limitation in that the detector area must be made correspondingly smaller in order to respond to individual fringes. Due to practical limitations on detector sensitivity, this in turn leads to a requirement for increased laser power, which leads to use restrictions due to safety considerations.
Another difficulty with the approach of increasing resolution by providing finer fringes is increased sensitivity to momentary loss of signal due to atmospheric particles or other objects interrupting the laser beam, as well as sensitivity to small vibrations.
Another disadvantage of fringe-counting approaches as previously proposed is that interpolation methods generally rely upon light amplitude information, which is difficult to obtain accurately. As a result of the constructive and destructive interference of the light waves, the fringes do not change instantly from light to dark as a function of distance. Rather, the change is gradual, and typical interpolation methods attempt to determine position between a pair of adjacent fringes by comparing amplitude at a particular point in question with expected maximum amplitude and expected minimum amplitude. Serious errors can be introduced into this interpolation by variations in atmosphere transmission which cause amplitudes to differ from expected values.